Add more validation checks

[sudoBash418]

AKA invent new, exciting ways to break everything
We can always use more validation checks - Some Smart Guy, probably.

I can't think of all the ways a challenge could realistically be broken or otherwise not work as expected.

If anyone wants to help out here, just let me know if you think of a check we could use and I'll add it - or make a PR and I'll relentlessly rewrite your code merge it in.

Relevant source code is mostly in cctk/sources/challenge.py at the moment.